System for sensing the approach of a moving missile to a target

ABSTRACT

A system for determining the trajectory of a missile and the minimum miss distance with respect to a target aircraft, comprises two transmitters on the aircraft each cooperating with four receivers on the aircraft. Each transmitter radiates a succession of pulses each having a very short duration of the order of 2 nanoseconds and each having a shape approximating to a single sine wave. The transmitted pulses are reflected from the missile and received by the receivers each of which is accurately time gated so that the received signal is sampled at a predetermined time delay after the radiation of each transmit pulse. A time delay corresponds to a particular missile range, and by gating at different delays the sampled signals indicate when the missile enters or leaves a plurality of range envelopes surrounding the target. Processing of the sampled signals enables the missile trajectory and minimum miss distance to be computed.

FIELD OF THE INVENTION

This invention relates to a system for sensing the approach of a movingmissile to a target, and more particularly (but not exclusively) to asystem for determining the trajectory and miss distance of a missileapproaching a target, for testing of the system. Such systems aregenerally called "scoring systems". The missile can be any high speedprojectile, such as an air-to-air missile, a ground-to-air missile, asea-to-air missile, a shell or a large bullet. The target can bestationary, airborne or on water but is preferably an aircraft, tank orother moving vehicle.

BACKGROUND TO THE INVENTION

Several scoring systems have been developed previously, which rely onradio transmission. These can be divided into those which requireequipment on both missile and target (called cooperative systems) andthose which are accommodated entirely on the target (callednon-cooperative systems) where the missile behaves as a passivereflector or scatterer. The invention is concerned with non-cooperativesystems.

U.S. Pat. No. 4,057,708 discloses a known non-cooperative system whichmeasures the minimum miss distance and three dimensional coordinates ofa missile trajectory with respect to a target in the form of anaircraft. This system uses pulses having a duration of 40 nanoseconds.Each transmitted pulse is derived by gating a continuously runningoscillating signal so the transmitted pulse consists of severaloscillations. The pulses reflected by the missile are received by areceiver on the aircraft, but accurate detection of missile presence isnot possible, firstly because of multipath effects due to reflectionfrom the aircraft skin and secondly because of the difficulty ofcorrectly identifying the received signal with the multiple scattererson the missile, such as nose fins or tail wings. The nature of thetransmitted pulse does not enable these different features to beresolved.

DISCLOSURE OF THE INVENTION

According to the invention there is provided a system for sensing theapproach of a moving missile to a target, comprising transmitting meanson the target for transmitting a succession of transmit pulses,receiving means on the target for receiving the pulses reflected by themissile, gating means on the target for sampling the reflected pulsesreceived at the receiving means during a succession of time windows eachof which is delayed with respect to the time of transmission of acorresponding transmit pulse by a predetermined time delay correspondingto the pulse travelling a predetermined distance from the transmittingmeans to the missile and thence back to the receiving means, so thatsampled signals from the receiving means are indicative of the missileentering or leaving a notional envelope surrounding the target at arange corresponding to said predetermined time delay, and processingmeans which are responsive to the sampled signals and which compute theapproach of the missile, wherein each transmitted pulse commences, froma level of no significant transmitted power in the frequency band widthof the pulse, with a rapid rise to a power peak, such that the sampledsignals are indicative of reflected pulses and not of reflections at thelevel of no significant power.

It will be appreciated that the envelope is an ellipsoid having thetransmitting means and the receiving means as the foci.

Each transmit pulse preferably has a duration of less than 4 nanosecondsand each pulse may have, after the rapid rise to the power peak, a rapidfall, continuing to rise to a power peak of opposite polarity so thatthe complete pulse is substantially sinusoidal in shape.

In the simplest system according to the invention, the transmittingmeans comprise a single transmitter and the receiving means comprise asingle receiver. This system provides a scalar result, that is it senseswhether the missile has penetrated the notional envelope without givingany more information about the spatial position of the penetration. Asystem providing a vector result, i.e. giving a three dimensionalindication of range requires at least three receivers located atdifferent positions on the target. Such a system can compute missiletrajectory and minimum miss distance. A practical vector system willnormally have more than the minimum number of three receivers, in orderto introduce some redundancy, and the preferred embodiment to bedescribed includes two transmitters each cooperating with acorresponding group of four receivers.

The gating means preferably produce gating pulses to sample thereflected pulses at the receiving means. The gating pulses are veryshort duration (typically of the order of 700 picoseconds) and areaccurately timed to be produced after a predetermined time delaycorresponding to a predetermined range of the missile. Directreflections from the missile will always arrive at the receiving meansbefore multipath signals. This, together with the fact that the sampledsignals will show no response prior to the response resulting fromdirect reflection of the initial peak of the very short durationtransmit pulse, avoids multipath problems and enables reflections fromdifferent parts of the missile to be resolved.

A system according to the invention will now be described, by way ofexample, with reference to FIGS. 1 to 16 of the accompanying drawings,in which.

FIG. 1 diagrammatically illustrates the system applied to a targetaircraft which has two transmitters and eight receivers,

FIG. 2 shows the pulses transmitted by the two transmitters,

FIG. 3 shows a standard radar pulse, for comparison with FIG. 2,

FIG. 4 illustrates how multipath effects originate,

FIG. 5 indicates how the inventive system overcomes multipath problems,

FIG. 6 is a plot of relative power against frequency for eachtransmitted pulse,

FIG. 7 is a block circuit diagram of the parts of the system on theaircraft,

FIG. 8 is a block circuit diagram illustrating one of the twotransmitters and one of the eight receivers,

FIG. 9 is a timing diagram illustrating range gating of the reflectedpulses at a representative receiver,

FIG. 10 shows how output signals from the receivers are processed,

FIG. 11 shows an idealized missile "signature", i.e. signal reflectedfrom the missile,

FIG. 12 shows the output from four range gates for each of tworeceivers,

FIG. 13 shows the three range gate envelopes associated with threereceivers cooperating with one transmitter,

FIG. 14 illustrates the part of the system on the ground,

FIG. 15 illustrates a missile track intersecting a family of range gateenvelopes,

FIGS. 16a to 16d are a block diagram of the system.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The system determines the trajectory and miss distance of a missileapproaching a target aircraft 10 (FIG. 1), for testing of the system.The system is non-cooperative, the aircraft 10 carrying two transmitters12, 14 and eight receivers 16, 18, 20, 22, 24, 26, 28, 30. Thetransmitter 12 is mounted on the top of the aircraft fuselage andcooperates with the four receivers 16, 18, 20 and 22 to monitor thehemisphere of space above the aircraft 10. The transmitter 14 is mountedon the underside of the aircraft fuselage and cooperates with the fourreceivers 24, 26, 28 and 30 to monitor the hemisphere of space below theaircraft 10. Each transmitter 12 or 14 transmits a succession ofelectromagnetic pulses which, on approach of the missile, are reflectedby the missile, the reflected pulses being detected by the correspondinggroup of four receivers. The signals detected by the receivers aretherefore representative of the trajectory of the missile, and thisinformation is transmitted by a telemetry transmitter 32, as indicatedat 34 in FIG. 1, to a ground-based telemetry receiver 36 linked by hardwiring 38 to a ground station 40.

FIG. 2 shows at 42 the pulses transmitted by the transmitter 12 and at44 the pulses transmitted by the transmitter 14. The pulses 42, 44 areidentical but they are interleaved, i.e. they are transmitted inalternate sequence, (to avoid ambiguity about the originatingtransmitter). Each pulse 42 or 44 has a duration of 2 nanoseconds andthere is a time T of 250 nanoseconds between successive pulses.

Each transmit pulse 42 or 44 is essentially a single cycle of sine waveand fundamentally has a power spectrum as in FIG. 6 but without thediscontinuity shown at 46. Individual parts of the spectrum may befiltered to avoid interference with other systems such as the dronecontrol filtering shown at 46 in FIG. 6. The pulse repetition frequencymight be varied from 1 MHz to 10 MHz depending on the application.

The important properties of each transmitted pulse 42 or 44 are that theduration of the pulse is sufficiently short to resolve multiplescatterers (such as the nose and tail) on the missile and alsosufficiently short to resolve multipath. The first peak of the waveformis very large compared to any energy (in the frequency band of thepulse) before this peak, which again distinguishes this system from atypical radar pulse (FIG. 3), so any received signal is unambiguouslydefined as emanating from this single cycle of energy.

FIG. 3 shows a typical radar pulse for contrast with FIG. 2. The pulsehas significant transmitted power in the region 48 before the main pulsepeaks at 50. Because the transmitted pulse of FIG. 3 has a series ofoscillations, the received pulse would be a jumble of peaks caused bymultipath interference and unresolved reflections from different partsof the missile.

FIGS. 4 and 5 show how the invention overcomes the problem of multipath,in the simplified example of the aircraft 10 having one transmitter 12and one receiver 22. Transmitted pulses 52, each having the form shownat 42 in FIG. 2, are reflected from the missile 54 to form a reflectedsignal 56 received by the receiver 22. The broken line 58 indicates anexample of a multipath, the signal from the missile being reflected bythe tailplane 60 of the aircraft 10 before reaching the receiver 22. Thevery short duration of each pulse 42 or 44, and the fact that the pulseis a near single cycle with a rapid rise to a power peak from a level ofno significant power in the bandwidth of the pulse, enables signals frommultiple reflections to be resolved, and also enables reflections fromdifferent features on the missile (e.g. nose and tail) to be resolved.

For example, FIG. 5 shows the idealized received signal in the inventivesystem as having significant amplitude at a first region 62 caused byreflection of a pulse from the missile nose, at a second region 64caused by reflection of a pulse from the missile tail, and at furtherregions 66, 68 caused by multipath reflections from the missile nose andtail respectively.

The airborne equipment of the system, i.e. the equipment on the aircraft10, is illustrated diagrammatically in FIG. 7.

Associated with each of the two transmitters 12, 14 are the antennas ofthe four receivers 16, 18, 20, 22; 24, 26, 28, 30. An upper transceiver70 generates the pulses transmitted by the upper transmitter 12 andprocesses the four waveforms received by the upper receivers 16, 18, 20,22. A lower transceiver 72 performs a similar function for the lowertransmitter 14 and lower receivers 24, 26, 28, 30.

The upper transceiver 70 includes a master clock in the timer sequencer82 (FIG. 8). The master clock produces a repetitive square wave therising edge of which triggers the upper transmitter 12 to produce apulse 42. The repetitive square wave is also passed to the timersequencer of the lower transceiver 72 and the falling edge of the squarewave triggers the lower transmitter 14 to produce a pulse 44.

The processed data from the two transceivers 70, 72 is passed to a dataformatter 74 where it is digitized and assembled into a serial datastream which is passed to a telemetry package 76 for transmission to theground by two telemetry antennas 78, 80 which are collectivelyequivalent to the transmitter 32 of FIG. 1.

FIG. 8 illustrates the composition of each transceiver 70 or 72.

The individual transceiver 70 or 72 is controlled by a timer-sequencer82 which by means of a stable oscillator triggers a transmit pulsegenerator 84 repeatedly and regularly. The transmit pulse is filtered bya band stop filter 86 to prevent interference with aircraft systems.This filtered pulse is then transmitted by the antenna of thetransmitter 12 (or 14). Each receiver (e.g. 16) receives a signal whichis filtered to reject the drone control (in this case a 20 MHz widefilter 88 at 400 MHz) and then filtered to band limit signals to 300 MHzto 1000 MHz by high pass and low pass filters 90, 92. The signal is thenamplified at 94 and sampled by gating means at precise delays afterpulse transmission in respective range gates or samplers 96, 98, 100,102. In this example there are four samplers per receiver at samplinginstants defined by the timer sequencer. The sampled waveforms are thenlow-pass filtered in filters 104 and passed to the formatter 74.

As indicated in FIG. 8, the components within the broken line boundary106 are repeated eight times, once for each of the eight receivers

The operation of the transceivers may be understood from FIG. 9 whichshows the sequence of events for a single receiver for each of the twotransmitters 12, 14. The delays of the four sampling pulses aftertransmission depend on the aircraft installation. In this example, therange gates are at 7.5 m, 15 m, 22.5 m and 30 m nominal rangecorresponding to a time interval of 50 nanoseconds between eachsuccessive sampling pulse. The interval between the two transmissions isdetermined by the requirement to receive the signal from transmitter 12at range 4 before the transmission from transmitter 14. The samplingprocess shown is repeated for each of the four receivers associated witha given transmitter. Hence, for each transmitted pulse 42 from thetransmitter 12, there are four signals from each of the four receivers16, 18, 20, 22, and for each transmitted pulse 44 from the transmitter14 there are four signals from each of the four receivers 24, 26, 28,30, making a total of thirty two signals.

The sampler 96 produces a first sampling pulse (shown as Range 1 in FIG.9) after a predetermined delay of 50 nanoseconds from transmission of apulse 42 from the transmitter 12, corresponding to the pulse covering atotal of 15 meters in its passage from the transmitter 12 to the missileand thence as a reflected pulse from the missile to the correspondingreceiver. Hence the reception of a reflected pulse at the receiverduring the time duration of this sampling pulse means that the missilehas penetrated a notional envelope or shell having a shape of anellipsoid with the transmitter and receiver as foci. The remainingsampling pulses (Range 2, Range 3, Range 4) are produced by the samplers98, 100, 102 at longer time delays (100, 150, 200 nanoseconds)corresponding to notional envelopes or shells which are larger in size.In each case, the sampling pulse is accurately timed and is of shortduration (700 picoseconds) so that the reception of a reflected pulsefrom the missile, during the very short time window of the samplingpulse, can be unambiguously resolved and reliably detected as themissile penetrating the predetermined envelope corresponding to the timedelay (50, 100, 150 or 200 nanoseconds) of the sampling pulse after thetransmitted pulse. A similar sequence of four gating pulses is producedafter similar time delays from transmission of a pulse 44 by transmitter14.

Referring to FIG. 10, the 2×16 audio frequency outputs from the twotransceivers 70, 72 (derived from the thirty two signals previouslymentioned) are passed to the formatter and filtered and amplified by thesignal conditioner so that the signal lies between ±5 volts and theindividual channels are passed to the analog-to-digital converter andconverted to 10-bit digital words. These words are then formatted into aserial bit stream for supply to the telemetry package 76 of FIG. 7.

FIG. 11 shows an idealized missile signature with separate scatteredwaveforms 108, 110 from the nose and the tail respectively. The durationand separation of the waveform depends on the length of the missile andthe illustrated case is for a missile approximately 2 meters long.

FIG. 12 shows the output from four range gates (5, 10, 15, 20 meters),for each of two receivers. There is a characteristic hyperbolic curveenclosing the missile approach and recede. The record illustrated showsconsiderable multipath echoes which the high resolution radar resolvesfrom missile scatter. The left hand edge 61 of FIG. 12 corresponds tothe instant before the misile arrives in the scoring zone. The missileis first detected at 63 at R×3 20 m and is then detected subsequently atshorter ranges 65, 67, 69. Separate reflections from nose and tail canbe seen at 71 and 73. The middle part of the signal corresponds tomultipath 75 and the missile then leaves each range gate in turn 77, 79,81, 83.

FIG. 13 shows a simplified system with a single transmitter 12 and threereceivers 16, 18 20. Each receiver and the transmitter define respectivenotional envelopes or shells 112, 114, 116 of ellipsoidal shape, havingthe transmitter and receiver as foci. Each shell corresponds to aparticular range gate at the corresponding receiver. No signal isobserved at a range gate until the missile 54 penetrates the shell.

The range gates shown are augumented by range gates at other ranges andother receivers to give 32 range gate shells surrounding the aircraftwhich the missile will intersect in a sequence determined by itstrajectory (see FIG. 15).

FIG. 14 shows the components of the telemetry receiver 36 and groundstation 40.

The telemetry data is received at a downlink receiver 118 and itsduplicate 120 and decrypted. (All the data from both receivers 118, 120is recorded on HDDR tape 122 for later analysis). The data stream isanalyzed by a frame synchronizer 124, which locates the framesynchronization word to divide the data into frames. One of thereceiving channels is selected by a source selector 126 for monitoringby the real-time system health monitor 128 which gives a go/no goindication for the system. A range interface 130 multiplexes range datainto the recording system such as time, firing event and voicecommunications.

For analysis, the data is replayed through a DMA interface controller132 which dumps the data into the memory of a Quick Look Processor 134.The Quick Look Processor 134 prints the data out on a chart recorder 136(a sequence of pictures as in FIG. 12 for each of the eight receivers)to enable a quick assessment of the trial.

The data is then transferred to computer compatible tape 138 fordetailed analysis by the track reconstruction facility.

In FIG. 15 a missile track 140 is shown intersecting a family ofenvelopes corresponding to different range gates of the system. Theimportant point is that the range gate intersections with respect todifferent receivers do not occur simultaneously. The missile trajectoryis modelled as a straight line whose starting point and velocity arechosen to produce the observed range gate crossing times. Where separatefeatures of the missile are detected (such as the nose and the tail),the different features move with a common velocity and the lengthbetween the points is constrained to a value around the physical length.

Thus the algorithm is minimized by the following function: ##EQU1##where: x_(i) Position of the detected missile feature at the time ofdetection

P Position of the transmitter which transmitted the pulse reflected fromthe missile

R Position of the receiver making the detection

d Range of the range gate making the detection

|| || Euclidean norm of the vector

The sum is taken over all detections of the missile.

FIGS. 16a-16d show the sequence of the system in block form. Referringto FIG. 16a, pulse generation 142 corresponds to the generation (at 84in FIG. 8) of the 2 nanosecond pulses at 2 MHz frequency. Filtering at144 removes part of the frequency spectrum interfering with the aircraftsystems and corresponds to filter 86 in FIG. 8. Transmission 146 isradiation of the sequence of pulses from the corresponding transmitter12 or 14, and reflection 148 is reflection from the missile. Reception150 is reception by the corresponding receiver, and filtering 152rejects interfering sources (such as drone telecommand), correspondingto filters 88, 90, 92 in FIG. 8. Amplification 154 corresponds to theprocess in amplifier 94 in FIG. 8, and sampling 156 corresponds to therange gate sampling performed at 96, 98, 100, 102 in FIG. 8.

Referring to FIG. 16b, filtering 158 rejects out of band noise(corresponding to filters 104) and conversion 160 converts the signalsfrom analog to digital format (corresponding to the analog to digitalconversion in formatter 74). Formatting 162 involves gathering the datafrom the thirty two receive channels in a serial bit stream andencryption 164 prepares the data for transmission to the ground station.The signals are then modulated at 166 and amplified at 168 before beingtransmitted to the ground at 170, corresponding to function 34 in FIG.1.

Referring to FIG. 16c, the data is received on the ground (172) by thetelemetry receiver 36 of FIG. 1. The remaining functions are carried outon the ground and are as indicated in the legends in the blocks of FIGS.16c and 16d.

The described system also measures the pitch and yaw of the missile,which it derives from the tracks of two independent scatterers on themissile (such as the nose and tail).

The described system may be extended to measure curved trajectories byadding more range gates to the system.

We claim:
 1. A system for sensing the approach of a moving missile to atarget, comprising transmitting means on the target for transmitting asuccession of transmit pulses, receiving means on the target forreceiving the pulses reflected by the missile, gating means on thetarget for sampling the reflected pulses received at the receiving meansduring a succession of time windows each of which is delayed withrespect to the time of transmission of a corresponding transmit pulse bya predetermined time delay corresponding to the pulse travelling apredetermined distance from the transmitting means to the missile andthence back to the receiving means, so that sampled signals from thereceiving means are indicative of the missile entering or leaving anotional envelope surrounding the target at a range corresponding tosaid predetermined time delay, processing means which are responsive tothe sampled signals and which compute the approach of the missile, andpulse generating means included in the transmission means, forgenerating said transmit pulses, each of which commences, from a levelof no significant transmitted power in the frequency bandwidth of thepulse, with a rapid rise to a power peak and a rapid fall such that eachtransmit pulse consists substantially of a single radio frequency cycleand such that the sampled signals indicate reflected pulses from themissile.
 2. A system according to claim 1, wherein said pulse generatingmeans comprises means for producing transmit pulses each having a timeduration less than 4 nanoseconds.
 3. A system according to claim 1,wherein the gating means comprises means for producing gating pulseseach having a duration corresponding to said time window.
 4. A systemaccording to claim 3, wherein the gating means comprises means forproducing gating pulses each having a duration of substantially 700picoseconds.
 5. A system according to claim 3, wherein the gating meanscomprises means operative to sample the reflected pulses at thereceiving means at a plurality of different time delays, so that thesampled signals from the receiving means are indicative of the missileentering or leaving a plurality of notional envelopes having sizescorresponding to the respective time delays.
 6. A system according claim1, wherein the transmitting means comprise a single transmitter and thereceiving means comprise a single receiver.
 7. A system according toclaim 6, wherein the single transmitter and single receiver are locatedat the same position, or closely adjacent one another, so that the oreach envelope is a sphere, or closely approximates to a sphere, centeredon the transmitter and receiver.
 8. A system according to claim 1,wherein the receiving means include a plurality of receivers located atdifferent positions on the target, each receiver being associated withthe same transmitter and each giving rise to a corresponding notionalenvelope for each time delay.
 9. A system according to claim 1, whereinthe transmitting means comprise two transmitters located at differentpositions on the target, the transmit pulses from the two transmittersbeing interleaved, so that pulses are produced from the transmitters inalternate sequence.